1. Field of the Invention
The present invention relates to a density detecting device and method, used for an image forming apparatus for forming an image by an electrophotographic process, for example, an electrostatic copying machine, for outputting density data utilized in adjusting the image forming conditions such as the amount of charge, the amount of exposure and the developing bias so as to keep the formed image high in quality.
2. Description of the Related Art
In the electrostatic copying machine, a copy image is formed in the following manner. Specifically, a real original which is put on a transparent platen is illuminated and scanned. Reflected light from the real original is introduced into a photosensitive drum which is rotated in synchronization with the illumination and scanning. As a result, the photosensitive drum is exposed. The surface of the photosensitive drum before the exposure is uniformly charged by a charger. An electrostatic latent image corresponding to the real original is formed on the surface of the photosensitive drum by selective charge elimination by the exposure.
The formed electrostatic latent image is developed into a toner image by a developing device to which toner is supplied from a toner hopper. The toner image is transferred onto copy paper by corona discharges in a transferring corona discharger. The copy paper on which the toner image has been transferred is introduced into a fixing device, where the toner is fixed to the copy paper, thereby completing copying.
An attempt to stably obtain an image high in quality in the above described electrostatic copying machine brings about the necessity of suitably adjusting the image forming conditions such as the amount of exposure and the amount of charge of the photosensitive drum, the developing bias and the amount of toner to be supplied to the developing device.
The image forming conditions are adjusted for each predetermined period, for example, at the time of maintenance. In adjusting the image forming conditions, a pure white or solid black pseudo original (a reference density original) arranged in a region other than a region where the real original is illuminated and scanned is experimentally illuminated, and a toner image corresponding to the pseudo original is formed. At this time, the amount of exposure, the surface potential, the density of the toner image on the surface of the photosensitive drum, and the like are detected, and the image forming conditions are automatically adjusted on the basis of the results of the detection. Specifically, in a case where the pure white pseudo original is illuminated to form a toner image, if so-called fog is detected on the basis of the detected toner image density, the amount of exposure is increased. On the other hand, in a case where the solid black pseudo original is illuminated to form a toner image, if it is judged that the density is insufficient on the basis of the results of the detection of the toner image density, toner is automatically supplied to the developing device from the toner hopper.
A reflection type photosensor which is constituted by a light emitting element and a light receiving element arranged opposed to the photosensitive drum is generally applied to the detection of the density of the toner image on the surface of the photosensitive drum. Specifically, light of a previously set amount is irradiated onto the photosensitive drum from the light emitting element, and density data corresponding to the amount of light reflected from the photosensitive drum is outputted from the light receiving element. Since the amount of the reflected light corresponds to the density of the toner image on the surface of the photosensitive drum, it is possible to detect the density of the toner image on the surface of the photosensitive drum on the basis of the above described density data.
At the time of initialization immediately after manufacturing the copying machine, two types of amounts of light to be irradiated, for example, an amount of light for low density and an amount of light for high density are set as an amount of light to be irradiated onto the photosensitive drum from the light emitting element in the reflection type photosensor. The amount of light for low density is an amount of light to be irradiated onto the photosensitive drum from the light emitting element when fog is detected. On the other hand, the amount of light to be irradiated for high density is an amount of light to be irradiated when a solid black is detected.
The reason why the amount of light to be irradiated is varied depending on a case where fog is detected and a case where a solid black is detected is as follows.
Specifically, when the fog is detected, the pseudo original on which a pure white image is formed is illuminated, whereby toner hardly adherers to the photosensitive drum. Consequently, the amount of light received by the light receiving element is relatively high. On the other hand, an output of the light receiving element is saturated if the amount of received light is increased. Therefore, the amount of light to be irradiated in the fog detection must be made relatively small so as to restrain the amount of light reflected from the photosensitive drum.
On the other hand, when a solid black is detected, the pseudo original on which a solid black image is formed is illuminated, whereby a large amount of toner adheres to the photosensitive drum. Consequently, most of light irradiated from the light emitting element is absorbed by the toner on the surface of the photosensitive drum, whereby the amount of light received by the light receiving element is relatively small. On the other hand, the light receiving element cannot detect a subtle change in the amount of received light if the amount of received light is small. Therefore, the amount of light to be irradiated when a solid black is detected must be made relatively large so as to increase the amount of reflected light.
FIG. 7 is a diagram showing the relationship between the density of a toner image on the surface of the photosensitive drum and density data outputted from the reflection type photosensor in a case where the amount of light for low density is set. Referring to FIG. 7, the density data outputted from the reflection type photosensor relatively linearly changes in a low-density region El, while hardly changing in a high-density region E2. That is, the reflection type photosensor can detect the change in density in the low-density region E1 with high precision in a case where the amount of light for low density is set. Therefore, it is possible to detect fog with high precision.
FIG. 8 is a diagram showing the relationship between the density of a toner image on the surface of the photosensitive drum and density data outputted from the reflection type photosensor in a case where the amount of light for high density is set. Referring to FIG. 8, the density data outputted from the reflection type photosensor hardly changes in a low-density region E1, while relatively linearly changing in a high-density region E2. That is, the reflection type photosensor can detect the change in density in the high-density region E2 with high precision in a case where the amount of light for high density is set. Therefore, it is possible to detect a solid black with high precision.
The predetermined amount of light for low density and the predetermined amount of light for high density, which are set at the time of the initialization, are also utilized for image forming condition adjusting processing performed for each predetermined time period. However, circumstances around the photosensor or status of the copying machine at the time of the image forming condition adjusting processing is different from that at the time of the initialization. Hence, toner image density is not always detected correctly, if the fixed amounts of light obtained at the time of the initialization are used at the time of the image forming condition adjusting processing. Consequently, the image forming conditions cannot be accurately adjusted, thereby making impossible to obtain an image high in quality.